Bottom trawling is a practice used by commercial fisheries around the world in which a large, heavy net is dragged along the ocean floor to scoop up everything in its path. Previous research has linked trawling to significant environmental impacts, such as the harvest of large numbers of non-target species, collectively termed "by catch," as well as destruction of shallow seabeds. Now, a new study published in Proceedings of the National Academy of Sciences finds this method is also resulting in long-term, far-reaching consequences in the deeper ocean and beyond.

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Trawling dates back to the 1300s, and it became widespread in coastal areas around the world after the industrialization of commercial fishing in the late-1800s. Bottom trawling targets commercially valuable species that live near the sea floor, such as cod, rockfish, and various kinds of squid and shrimp. Gear varies depending on the fishing outfit, but nets can be nearly as large as a city block and scoop thousands of fish and other marine animals in a single drag.

Bottom trawling has one of the highest bycatch rates of all commercial fishing practices. In the North Pacific, the practice accounts for 18 percent of annual groundfish harvests, and 82 percent of the region's discarded by-catch. At times, bycatch accounts for upwards of 90 percent of a net's total catch.

In addition to directly killing many fish and other marine species, studies have shown that bottom trawling is very destructive to the seabed. It dislodges sediment, which destroys the habitat of ground-dwelling organisms, makes the water more opaque and unsuitable for many species, and releases pollutants and carbon that had been trapped below the seafloor.

As populations of many fish species dwindle due to intensive commercial fishing effort, bottom trawling outfits are searching for new fishing grounds in ever-deeper regions of oceans around the world. However, this new study indicates that deeper ocean bottoms are also being affected by trawling, as the nets destroy delicate seafloor ecosystems at a level akin to desertification.

"Cumulatively, the impacts of trawling on the sediment structure, the benthic biodiversity, and the most basic of all of the nutritional resources in these deep-sea sedimentary ecosystems resemble the catastrophic effects caused by man-accelerated soil erosion on land, and the general environmental deterioration of abandoned agriculture fields exposed to high levels of human impact," write the researchers, from various institutions in Italy and Spain.